Inwardly rectifying potassium channels (Kir) are critical regulators of cellular excitability. The property of inward rectification ensures that these channels (i) are closed at depolarized potentials, which allows for stable plateau potential and (ii) open at the resting potential, thus stabilizing it. Although the phenomenon of the strong inward rectification of Kir channels has been studied extensively for several decades, lack of specific pharmacological tools for their modulation and unknown mechanism of rectification precluded detailed investigation of their role in cellular excitability. Now, with the recent discovery of the mechanism of inward rectification and availability of genetic means of Kir channels manipulation, this project seeks to understand the regulation of Kir channels underlying the major inwardly rectifying current of the heart (IK1) and their role and importance in cardiac excitability. The following Specific Aims are proposed to achieve the overall goal of the project: (1) To understand molecular basis of inward rectification in the mouse heart. (2) To understand the role of inward rectification in cardiac excitability by expressing mutant Kir channels in the mouse heart. (3) To understand the regulation of Kir channels by intracellular polyamines (PA) in the mouse heart with altered PA biosynthesis. (4) To develop an interactive computer model of cardiac action potential incorporating the latest findings on the mechanisms of PA-induced rectification for the analysis of the data obtained during this project. This project will utilize a broad spectrum of current and developing technologies - molecular biological and genetic, physiological and biophysical as well as computer modeling - to advance our knowledge of cardiac excitability that will ultimately lead to the development of new therapies for the treatment of cardiac malfunction.